Does he kind of hunker down on all four paws, act like he's trying to throw up and sound kind of like a low cough with no phlegm?
Because you're worried, go ahead and get him to the vet.
(If you can afford it, might not hurt to do follow-up blood work to see how things are going, but ask the vet - and did he have round worm? May want to remind them to do a quick poop check as long as you're there).
But I'm betting on hairballs. Scared the heck out of us too at first. Flowerbelle and Lazlo have constant problems with them - none of the rest do. Flowerbelle HATES taking any of the meds for it, so the vet said go ahead and just use the Friskies treats for hairballs for her. They're working for her! For Lazlo, we got some natural thingy from Petsmart - he loves it and laps it up. Usually you give it every day until they pass the hairball (which for both of them means barfing them up - eeeewww!) - we give it to them 2x - 3x per week after that to help keep the hairballs at bay.
We also use a curry comb kind of thing on them to get out excess hair. Well - we use it on all the kitties, and it's good for me too as I'm allergic to cats. But you wouldn't believe how much hair comes out EVERY DAY!